''Shall we get undressed?'' This is what I want to ask, the first provocation written on my list, but she looks so decent, so welcoming, that the thought of it is crass. I am here to meet the actress, not the sex therapist she plays in her latest movie.

It's not quite my first time, but she guides me gently through the process, so familiar to her after a lifetime in pictures, many of them, since her Oscar-winning performance opposite Jack Nicholson in As Good As It Gets, the sort of big-budget entertainments that are promoted this way, one interview after another in a five-star suite. She knows exactly how much to give and when to hold back.

''You get to say what feels good and what doesn't,'' she tells me. ''Listening to the other person is a big deal. Loving yourself is as important as being loving to the other person. Say what you want, say what you don't want, have a sense of humour and get out of there if it's not working.''

The Sessions is based on an essay, On Seeing a Sex Surrogate, by a writer named Mark O'Brien, who spent most of his life in an iron lung after being struck down by polio at the age of six. Unable to move his arms or legs or breathe independently for more than a couple of hours at a time, he tapped out his articles with a pencil held in his mouth. When he was commissioned to write about sex and disabled people, it forced him to confront his own feelings.

''Being disabled myself, but also being a virgin, I envied these people ferociously,'' he wrote. ''It took me years to discover that what separated me from them was fear - fear of others, fear of making decisions, fear of my own sexuality, and a surpassing dread of my parents.'' After receiving the blessing of a priest at his Catholic church, he set out to lose his virginity.

John Hawkes portrays O'Brien as a self-aware wit, attractive to women every way but physically and all the more frustrated for it. Anyone who has only seen his Oscar-nominated turn in Winter's Bone, as a menacing methamphetamine dealer named Teardrop, is in for a surprise.

The film is an independent production, directed by Australian Ben Lewin, a veteran of British television, but after it won the Audience Award at the Sundance Film Festival, Fox Searchlight bought the distribution rights and has high hopes that its stars will be in the running for statuettes when Oscar season comes around.

Hunt plays Cheryl Cohen-Greene, the sex therapist who helped O'Brien get his wish. ''She's still working, still sex-surrogating, grandmother, cancer survivor, happily-married woman,'' Hunt says. ''As you can imagine, she's not shy or withholding. She told me that when she walked into the room she was really freaked out because she had worked with disabled people before but no-one this disabled, so she wondered whether she would be able to do it and it was very important not to show that on her face.''

The two women talked at length about Cohen-Greene's experiences. Hunt asked her to read the script out loud, to internalise her Boston accent. ''I also had her come over and touch my boyfriend's feet so that I could see what sensual touch means: is it scratching, massaging, something in the middle?''

Naturally, in the role of a physical therapist whose job is to loosen inhibitions, Hunt spends much of the movie naked, and from the first, perky ''shall we get undressed?'' she seems completely at ease with her body. It helps that she is in phenomenally good shape for a 49-year-old woman and that vanity is apparently not one of her vices. She has never had plastic surgery or botox, despite working in an industry that all but demands it for actresses in middle-age. On the day we meet, she looks flawless to the last follicle, having evidently been treated to a high-class pre-interview touch-up.

Four years ago, Hunt wrote and directed Then She Found Me, playing a teacher in the grip of a mid-life crisis. She opted to perform without make-up, under harsh lights that highlighted every wrinkle. Baring all for The Sessions was easy by comparison. ''First I made the cameraman lay down and I, fully clothed, got on top of him, and said, 'Where will the camera be, when will we show how much?' I didn't want to be protecting myself while exposing myself. I wanted to protect myself first and then expose myself in the movie,'' she says.

In a film called The Waterdance, Hunt's character had a sexual encounter with a young author paralysed in a hiking accident played by Eric Stoltz. ''I asked myself, 'Why are those the two movies I said yes to?' And I think it's because, by definition, the fake elegant thing that movies do with sex can't happen, so something truer has to happen,'' she says. The sex scenes in The Sessions are some of the least self-consciously racy ever filmed. ''It was harder in some ways, but less ridiculous,'' Hunt says. ''You know: 'action', the hands grip, 'action', the underwear slips off. This was challenging because it was vulnerable, but those sex scenes are weird. I haven't done a lot of them but that feels more like being in a porno than this did.''

I ask if she shares Cohen-Greene's uncomplicated, guilt-free attitude to sex. ''I don't have terrible hang-ups, but I'm not quite the pure joy-emanating machine that she is,'' Hunt says. ''I hope my kids have a big scoop of that when they grow up.'' Her daughter, Makena Lei, is eight years old. Hunt has been thinking about what to tell her, when to tell her and how. ''I know that it's got to happen soon or she's gonna hear it somehow, and I don't want her to hear it from anybody but us.''

O'Brien's essay is frequently funny, but it concludes on a despairing note: ''I feel no enthusiasm for the seemingly doomed project of pursuing women. My desire to love and be loved sexually is equalled by my isolation and my fear of breaking out of it.'' At the time of writing it, he was unaware of a real-life happy ending that awaited him. For Lewin, who also suffered polio as a child, this is what transformed an intriguing subject into a marketable movie. At Sundance, the audience gave him a standing ovation as he made his way to the front on crutches.

''At the main screening, they were laughing so loud that they couldn't hear the dialogue,'' Hunt says. ''Then they got scared, because they were so with Mark that they were more afraid I would take my clothes off than titillated by it. And at the end, they all stood up for this filmmaker in his 60s. It was thrilling.''

■ The Sessions is screening in Australian cinemas now.